U.S. Pat. No. 3,196,617 discloses a way whereby air-borne contamination can be prevented from being communicated to a reservoir of a master cylinder. In this apparatus, a diaphragm which seals the reservoir from tha atmosphere will follow the brake fluid level as changes occur therein. Under some conditions, a pressure differential will occur across the diaphragm which will move the diaphragm into the brake fluid. When the diaphragm is moved in the brake fluid, a portion of this fluid will be displaced upward along the diaphragm and the walls of the reservoir creating a false fluid level indication of the quantity of brake fluid within the reservoir.